For nearly three quarters of a century, metallurgists have been using additives such as lead, sulfur and selenium to enhance the machinability of stainless steels. The eternal challenge has been to keep the levels of such additions low enough that they would not seriously compromise any of the inherent properties desired of these alloys.
The world's first free-machining stainless steel, invented by Carpenter's Frank Palmer in 1928, was a straight chrome grade with sulfur added. It was the forerunner of today's Type 416 stainless.
Sulfur and phosphorus were both added to make Carpenter Type 303 stainless, the first free-machining chrome-nickel stainless, in the early thirties.
Close control of chemistry contributed to the next major improvement in the machinability of stainless steels, although sulfur additions remained key to alloy performance. Carpenter's Project 70® series of stainless steels, developed in the mid-sixties, continued to keep sulfur levels below industry-accepted limits. Project 70 grades like Type 303 and Type 416 stainless, considered to be true free-machining grades, contain a 0.15% minimum sulfur content. Other Project 70 grades such as Type 304, Type 316, Type 321, Type 347 and Custom 630 (17Cr-4Ni ) stainless were re-sulphurized up to a much lower 0.03% maximum to enhance their machinability.
The most recent major advance in the evolution of stainless steels with improved machinability characteristics occurred in the mid 90's with the introduction of Carpenter's Project 7000® series of stainless alloys. Machining properties of these stainless steels were improved by carefully modifying the compositions, and by managing the delicate relationship between chemistry and modern steel processing methods. All of the grades in this series meet industry requirements for composition, corrosion resistance and mechanical properties, while providing significantly better machinability.
The alloy to be discussed in this article, Project 7000 15Cr-5Ni stainless (UNS S15500), stemmed from a specific need for enhanced machinability in the aerospace industry, and other industries as well. It is the newest in the Project 7000 series of stainless steels, offering advantages derived primarily from tight compositional and manufacturing controls.
During a recent plant visit, a major manufacturer of aerospace components indicated to us that his shop did not have the capacity to keep up with the demand for parts he was machining from conventional 15Cr-5Ni alloy. The shop had such a large backlog of orders that the manager was considering buying more machine tools to expand production capacity, but was struggling with the problem of capital to finance the purchase.
Wait a minute before you go to that expense, suggested the Carpenter visitor. Had he explored the possibility of using the same alloy, with (1) improved machinability properties, that would (2) meet industry specifications and (3) give him substantially more throughput using existing equipment (4) without incurring the prohibitive cost of buying new machinery? His negative reply was all that was needed to inspire final development and commercialization of the more machinable 15Cr-5Ni alloy variation.
The commodity 15Cr-5Ni alloy has been used extensively by the aerospace industry for parts and components that can benefit from its good combination of high strength, fracture toughness and corrosion resistance. Its application, however, has been limited by the fact that it is difficult to machine.
Recognizing the unfulfilled need, Carpenter's Research and Development Lab invented its Project 7000 version of the 15Cr-5Ni alloy that offers superior machinability and, with it, the opportunity to reduce part costs, cut cycle time and increase productivity. The specially-balanced alloy does this while meeting all requirements of Aerospace Material Specification AMS 5659 covering bars, wire, forgings, rings and extrusions, as well as ASTM A564. While providing a distinct advantage in machinability, the Project 7000 alloy variation retains all the corrosion resistance and mechanical properties inherent in the original stainless.
This upgraded stainless steel, which offers potential also for increased capacity and longer tool life, is available as a "drop-in" replacement for the conventional 15Cr-5Ni stainless in applications where improved machining productivity is desired. There is no need for a parts producer to qualify or requalify the improved alloy since it meets all the required specifications.
Machining data generated in the laboratory, along with data from Beta-site tests verify the improved machinability of the Project 7000 15Cr-5Ni alloy. Lab tests comparing the machinability of the modified alloy with the commodity grade are shown in Fig. 1. Parts were machined in the solution annealed condition and two age hardened conditions (H1025 and H1150). Both alloys are most commonly supplied by the manufacturer, and subsequently machined in the annealed condition.
A screw machine test was employed in which a standard part was made repeatedly at a fixed speed and feed until the test was terminated because of tool wear. High speed steel inserts were used for tooling. Under these conditions, performance ratios were calculated based on the relative number of parts produced when machining both alloys. Fig. 1 Performance ratios compare machinability of the Project 7000 15Cr-5Ni alloy with that of the commodity 15Cr-5Ni alloy in laboratory testing. Speeds and feeds utilized during testing depended upon the specific heat-treat condition.
Tests showed that 11 times more parts were produced from the Project 7000 stainless than from the conventional 15Cr-5Ni alloy before the predetermined amount of tool wear was reached. In the H1025 and H1150 conditions, the Project 7000 grade produced 3½ times, and 2 times more parts than the commodity alloy, respectively.
These ratios are used for comparative purposes only since they do not necessarily relate to actual component machining. More relevant parameters such as cycle time reduction and productivity improvements must be generated by the alloy user under conditions for actual component production.
In one highly successful production application, Sturm Ruger & Co., Inc., Southport, CT, used Project 7000 15Cr-5Ni alloy as a replacement for the conventional 15Cr-Ni alloy for the barrel in its new Super Redhawk .454 Casull. This is one of the most powerful commercially available revolvers in the world.
Designers were concerned with the throat erosion that might occur when the big cartridge would exit the chamber and slam into the interior surface of the barrel. The conventional 15Cr-5Ni stainless steel used for the barrel initially met all the design requirements for strength and corrosion resistance but one. It was very difficult to machine.
It took Ruger 28 minutes to gun drill a 0.480" dia. hole in a 1¼" OD x 19" -long bar (later hammer-forged over a mandrel and cut into three 7½" -long gun barrels) at the slow rate of 0.71 IPM. After switching to the Project 7000 version of the 15Cr-5Ni alloy, Ruger was able to reduce its gun drilling cycle time by 20% while improving tool life significantly. With its excellent transverse mechanical properties, the replacement alloy was able to resist the every high stresses produced by the large cartridge.
Preliminary results with aerospace applications also have been impressive. One parts manufacturer, machining a part that required multiple operations including milling, boring, turning, drilling and thread rolling, achieved a 145% improvement in productivity.
Another company, doing a simple turning operation, cut cycle time by 1/3 and increased tool life by 50%. Still another parts producer, in milling the alternative alloy, was able to triple speeds and feeds, and increase depth of cut by a factor of 5 with no burning of the cutting tools.
Project 7000 15Cr-5Ni stainless can be considered for a wide variety of applications in the aerospace industry. With its high strength and hardness, along with excellent corrosion resistance, the alloy is a candidate material for aircraft structural components such as rod-end bearings, actuators, landing gear structure and engine parts such as brackets and thrust reverser components.. It also may be considered as a more machinable version of the conventional 15Cr-5Ni stainless which has been used for industrial applications such as gun barrels, valve parts, fittings, fasteners, shafts, gears and process equipment.
The Project 7000 alloy is a vacuum re-melted, precipitation hardening, martensitic stainless steel that will allow shops to push their machining speeds and feeds to higher levels, challenging the material for best results. Machinability may be maximized with assistance from the alloy developer, since results ultimately depend on the specific application, equipment and operator skills. Without technical assistance, the shop interested in trying the new stainless can do well by machining first at its own comfort level, then ramping up the processes to find out how far it can go and benefit.
Typical machining speeds and feeds for the replacement alloy are tabulated in Fig. 2. The data for turning in both charts represent conservative recommendations for initial setup. Higher speeds and feeds may be obtainable depending on the machining environment. Both tests and on-job experience has shown that bar size has no effect on the machinability of this material.
Fig 2. Typical machining speeds and feeds for turning Project 7000 15Cr-5Ni alloy, using single-point and box tools in the upper table and cut-off and form tools in the lower table.>
In addition to improved machinability, the Carpenter alloy also can be headed, rolled and otherwise formed by cold working. It can be forged by heating the work piece uniformly to 1066/1121°C (1950/2050°F) and held one hour at temperature before forging in accordance with a recommended schedule. The alloy can be welded by the shielded fusion and resistance welding processes.
The general corrosion resistance of Project 7000 15Cr-Ni alloy approaches that of Type 304 stainless, and is similar to that of Carpenter 15Cr-5Ni and Custom 630 (17Cr-4Ni) stainless steels in most media. It offers good resistance to stress corrosion cracking by age hardening at temperatures of 552°C (1025°F) and higher.
The upgrade alloy displays excellent resistance to oxidation up to approximately 593°C (1100°F). Long-term exposure to elevated temperatures can result in reduced toughness in precipitation hardenable stainless steels. The reduction in toughness can be minimized in some cases by using higher aging temperatures.
The case for the Project 7000 material as a "drop-in" replacement alloy for the basic 15Cr-5Ni grade is also reinforced by the fact that its mechanical properties are comparable to those of the original alloy. The mechanical properties of both alloys conform with AMS 5659 specifications.
Typical room temperature mechanical properties for the Project 7000 alloy, in various heat treated conditions, are shown in Fig. 4. While these values reflect properties for 3"-sq. bar, there is very little variation in properties for bars both smaller and larger.
It is worth noting that in the H925 condition, Project 7000 15Cr-5Ni stainless retains excellent longitudinal and transverse fracture toughness while meeting a frequently specified 1240 to 1380 MPa (180-200 ksi) ultimate tensile strength requirement. At even higher aging temperatures, fracture toughness is improved further. Due to the homogeneity of the Project 7000 15Cr-5Ni alloy version, there is very little difference between longitudinal and transverse properties.
Project 7000 15Cr-5Ni stainless steel is available from Carpenter in billets, wire, bars and wire rods.
Fig. 4 Typical room temperature mechanical properties of 76.2mm (3"-sq.) bar of Project 7000 15Cr-5Ni alloy
By James M. Dahl and James W. Martin
Carpenter Technology Corporation